1. Field of the Invention
The invention relates to one-component blends comprising alkoxysilane-terminated polymers which cure at room temperature under the action of atmospheric moisture to form low-modulus compositions.
2. Description of the Related Art
Polymer systems which possess reactive alkoxysilyl groups have been known for a long time. In the presence of atmospheric moisture these alkoxysilane-terminated polymers are capable even at room temperature of undergoing condensation with one another, with the elimination of the alkoxy groups. Depending on the amount and structure of the alkoxysilane groups, the condensation products are principally long-chain polymers (thermoplastics), relatively wide-meshed three-dimensional networks (elastomers) or else highly crosslinked systems (thermosets).
The polymers concerned may be either alkoxysilane-terminated polymers with an organic backbone, such as polyurethanes, polyesters, polyethers, etc. described inter alia in EP-A-269 819, EP-A-931 800, WO 00/37533, U.S. Pat. No. 3,971,751 and DE-A-198 49 817, or polymers whose backbone is composed wholly or at least partly of organosiloxanes, described inter alia in WO 96/34030 and U.S. Pat. No. 5,254,657.
In accordance with the countless possibilities for designing such silane-terminated polymer systems, the properties of the uncrosslinked polymers or of the polymer-containing mixtures and also the properties of the crosslinked compositions (hardness, elasticity, etc.) can be adjusted. Correspondingly diverse, therefore, are the possible uses of such silane-terminated polymer systems. For example, they may be used for preparing elastomers, sealants, adhesives, elastic adhesion systems, rigid or flexible foams, and a very wide variety of coating systems, and in the medical sphere, for example for impression compounds in the dental sector. These products can be applied in any form, such as by brushing, spraying, pouring, pressing, trowelling, etc.
In many systems a substantial disadvantage lies in the contradictory effect of chain length or molecular weight of the polymer used and the processing properties in terms of the viscosity. High molecular weights are not only of interest owing to the higher mechanical strength associated with them but are also an important prerequisite for the preparation of low-modulus elastomers, as are required in particular in sealants. Where polymers of lower viscosity can be used in such compositions, the adjustment of the processing properties becomes much simpler and more flexible.
For example, a silicone polymer must be used with a viscosity which is as high as possible in order for the cured product to achieve the right properties in terms of ultimate tensile strength and elasticity for use in construction sealants. State of the art here is a polymer having a viscosity of at least 80 Pas. Such a polymer, however, gives rise to adverse properties in the paste, such as stringing, poor smoothability, and high plasticizer content. For setting these properties the ideal would be a polymer having a viscosity of not more than 20 Pas.
This tendency is even more pronounced in the context of the use of silane-terminated polyurethanes. Here, it is well-nigh impossible to prepare low-modulus compositions without significantly impairing the mechanical properties, generally as a result of additions of plasticizer.
A great advantage, therefore, would be alkoxysilane-terminated polymer systems which on curing bring about not only crosslinking but also a chain extension of the polymers. In order to reduce the crosslinking densities it is common to incorporate difunctional silanes into the polymers. Since the reactivity of such compositions is generally much lower, it is necessary to raise the amount of catalysts (usually containing tin) sharply. In this context it would be particularly advantageous to be able to use not only difunctional silanes but also monofunctional silanes, which are able to bring about chain extension exclusively. The known monofunctional silane end groups, based on silanes having trimethylene spacers between the organic functional unit and the silicon atom, however, are so slow to react that they generally function as “dead” chain ends.
DE-A 2543966 and DE-A 2445220 describe the use of polymers containing monofunctional alkoxysilane endgroups for preparing one- and two-component polysiloxane-polyurethane copolymers. Here, isocyanate-terminated polyurethanes are reacted with aminomethylmonoalkoxysilanes (especially cyclohexylaminomethyldimethylethoxysilane) and the polymers obtained are crosslinked with crosslinker units and OH-terminated polysiloxanes to form elastomers.
One of the disadvantages of this system is the fact that the reaction is tied to the use of isocyanate-terminated prepolymers. Polymers of this kind are normally prepared by reacting excess diisocyanates with polyols. As a result of the incorporation of the urethane groups, but in particular as a result of the large number of side reactions occurring when an excess of isocyanate is used (biuret formation, formation of urea bonds through hydrolysis of the NCO groups and condensation with further NCO groups, etc.), this generally leads to very viscous polymers. The incorporation of the aminosilanes to form urea groups reinforces this negative effect. To compensate for it these polymers must usually be synthesized with relatively low molecular weights. That, however, adversely affects the elastic behavior of the cured products. In the case of silanes with a functionality of two or three, only very brittle compositions of high modulus are generally obtained. The incorporation of monoalkoxy endgroups is certainly advantageous here. However, the modulus of these compositions is always very high and so presents problems for applications in the construction sealant sector.
As depicted in EP-A-931800, the incorporation of isocyanatosilanes results in a marked improvement in the properties of the sealants as compared with corresponding polymers prepared by way of aminosilanes. However, it has not proven possible to date to use monofunctional silanes here, since the silanes customary at present, with trimethylene spacers, as already mentioned above, have a reactivity which is much too low.